Electrochemical cells invariably comprise at their fundamental level a solid or liquid ion conducting electrolyte and two electrodes, the anode and cathode, at which the desired electrochemical reactions take place. Electrochemical cells may be found in a range of devices, for example fuel cells, batteries, sensors, electrodialysis reactors and electrolytic reactors for a diverse range of applications including the electrolysis of water, chemical synthesis, salt splitting, water purification, effluent treatment, and metal finishing among others.
A fuel cell is an energy conversion device that efficiently converts the stored chemical energy of its fuel into electrical energy by combining either hydrogen, stored as a gas, or methanol stored as a liquid or gas, with oxygen to generate electrical power. The hydrogen or methanol are oxidised at the anode and oxygen is reduced at the cathode. Both electrodes are of the gas diffusion type. The electrolyte has to be in contact with both electrodes and may be acidic or alkaline, liquid or solid, in nature. In proton exchange membrane fuel cells (PEMFC), the electrolyte is a solid ion conducting, or more specifically a proton conducting, polymer membrane, commonly based on copolymers of perfluorosulphonic acid and tetrafluoroethylene, and the combined structure formed from the membrane and the two gas diffusion electrodes is known as the membrane electrode assembly (MEA).
Conventionally, solid ion conducting membrane electrolytes useful in fuel cells and other devices are selected from commercially available membranes, for example perfluorinated membranes sold under the trade names Nafion.RTM. (E.I. DuPont de Nemours and Co.), Aciplex.RTM. (Asahi Chemical Industry) and Flemion.RTM. (Asahi Glass KK). For application in the PEMFC they are typically below 200 .mu.m in thickness to ensure a high level of ionic conductivity. One of the problems experienced with these conventional proton conducting membranes used for PEM fuel cell construction, is the dimensional changes that occur as the level of water content (hydration) of the membrane changes. This is a particular problem during fabrication of the MEA as the stresses produced by changes in hydration during the conventionally employed thermal bonding process, can be large enough to break the bond between the catalyst and the membrane, or the catalyst and the substrate. Furthermore, the dimensional changes that occur due to the changes in the level of hydration of the membrane lead to considerable difficulties in handling membranes during the fabrication of MEAs, particularly large area MEAs in excess of, for example, 500 cm.sup.2. The thinner the membrane, the more difficult the handling becomes. With thicker types of membrane (eg&gt;350 .mu.m) developed for other applications, it has been possible to incorporate `macro` reinforcing materials such as woven polytetrafluoroethylene (PTFE) to minimise such dimensional changes. However, these thicker materials have too low an ionic conductivity to be of use in the PEMFC. U.S. Pat. No. 5,547,551 assigned to W. L. Gore & Associates Inc., describes the fabrication of ultra-thin composite membranes, below 25 .mu.m in thickness, comprising proton exchange polymer material incorporated into an expanded porous PTFE membrane. According to Kolde et al., Electrochemical Society Proceedings Vol. 95-23, p193-201 (1995), the composite membrane has considerably improved dimensional stability compared to the conventional non-reinforced membranes. The material has, however, a higher specific resistance (lower ionic conductivity) than an unmodified pure proton conducting membrane such as Nafion.RTM.117 by a factor of at least two.
The higher specific resistance of the above composite membrane means that in practice it has to be much thinner than the equivalent pure proton conducting membrane to maintain the same overall conductivity and thus cell performance. However, reducing the thickness of the composite membrane reduces the advantages that a composite membrane can provide. For example, there is a limit to the extent to which the thickness of the membrane can be reduced since as the membrane is made thinner, the durability and longevity can decrease, and reactant gas cross-over through the membrane is more liable to occur, both of which lead to a reduction in the cell performance. Furthermore, the problems associated with dimensional stability and handlability for MEA fabrication can be exacerbated with thinner membranes.
E.I. DuPont de Nemours and Co. (WO95/16730) describe a process for making a reinforced substantially non-porous membrane with satisfactory mechanical strength and very low resistance to ionic conduction which approaches that of very thin, unreinforced perfluoro ion exchange polymer membranes. The composite membrane utilises a porous hydrocarbon substrate, such as a polyolefin, and on which at least one side is coated with an ion exchange film formed from a fluorinated polymer.
It is therefore an object of the present invention to overcome the disadvantages of conventional pure and composite membranes, by providing a novel composite ion exchange membrane with improved dimensional stability and handlability, and in which the ionic conductivity and reactant gas cross-over have not been compromised compared to a conventional unreinforced ion exchange membrane of the same polymer and comparable thickness. A further object of the present invention is to provide a process for the manufacture of the composite membrane of the invention, in particular a process that is capable of producing composite membranes in high volumes and with high yields and at low unit cost, and preferably as a single continuous process. A still further object is to provide a process for preparing an MEA in high volumes and with high yields and at low unit cost.
Accordingly, the present invention provides a composite membrane comprising a porous substrate of randomly orientated individual fibres and at least one ion conducting polymer, characterised in that the ion conducting polymer is embedded within the porous substrate. Alternatively, there is provided a composite membrane comprising a plurality of fibres randomly combined to form a porous substrate and at least one polymeric material, characterised in that the polymeric material is embedded within the porous substrate.
The porous substrate typically has at least 50%, suitably at least 75% of the individual pore sizes being greater than 1 .mu.m in at least one direction, although a porous substrate wherein some of the pores are less than 1 .mu.m in all directions is within the scope of the invention. Suitably, for applications in fuel cells, the total thickness of the membrane is less than 200 .mu.m and preferably less than 100 .mu.m.
The fibres within the substrate are normally randomly orientated in the x and y direction (in-plane) producing a two dimensional isotropic structure. Additionally, random orientation in the z direction (through-plane) can be introduced with the inclusion of very short fibres, typically lengths of less than or equal to 0.2 mm or very fine fibres, typically of diameters less than or equal to 1 .mu.m. Fibres which are suitable for use in the present invention include glass, polymer, ceramic, quartz, silica, carbon or metal fibres. Fibres of carbon or metal would need to be electrically insulated prior to being formed into the membrane. Suitably, if polymeric fibres are used, the fibres are not polytetrafluoroethylene (PTFE) or polyethylene fibres. Suitably, the fibres are of glass, ceramic, quartz, silica, carbon or metal and preferably of glass, ceramic, or quartz. The fibres are typically of diameters in the range of 0.1 .mu.m to 50 .mu.m, preferably of 0.2 .mu.m to 20 .mu.m and with lengths from 0.05 mm to 300 mm, suitably 0.5 mm to 150 mm. FIG. 1 shows a micrograph of a typical substrate formed from glass fibres containing only one diameter of glass fibre, obtained using a scanning electron microscope, and which clearly show a substrate of randomly orientated individual fibres lying in the x and y directions only. FIG. 2 shows a substrate with a range of fibre diameters with the finer fibres giving rise to fibres lying in the z direction.
For PEM fuel cell applications, the ion conducting polymer is a proton conducting polymer, examples of such polymers being well known to those skilled in the art. More than one proton conducting polymer may be present and/or a non-proton conducting polymer may also be included in the novel membrane of the present invention.
The proton conducting polymers suitable for use in the present invention may include, but are not limited to:
1) Polymers which have structures with a substantially fluorinated carbon chain optionally having attached to it side chains that are substantially fluorinated. These polymers contain sulphonic acid groups or derivatives of sulphonic acid groups, carboxylic acid groups or derivatives of carboxylic acid groups, phosphonic acid groups or derivatives of phosphonic acid groups, phosphoric acid groups or derivatives of phosphoric acid groups and/or mixtures of these groups. Perfluorinated polymers include Nafion.RTM., Flemion.RTM. and Aciplex.RTM. commercially available from E. I. DuPont de Nemours (U.S. Pat. No. 3,282,875; 4,329,435; 4,330,654; 4,358,545; 4,417,969; 4,610,762; 4,433,082 and 5,094,995), Asahi Glass KK and Asahi Chemical Industry respectively. Other polymers include those covered in U.S. Pat. No. 5,595,676 (Imperial Chemical Industries plc) and U.S. Pat. No. 4,940,525 (Dow Chemical Co.)
2) Perfluorinated or partially fluorinated polymers containing aromatic rings such as those described in WO 95/08581, WO 95/08581 and WO 97/25369 (Ballard Power Systems) which have been functionalised with SO.sub.3 H, PO.sub.2 H.sub.2, PO.sub.3 H.sub.2, CH.sub.2 PO.sub.3 H.sub.2, COOH, OSO.sub.3 H, OPO.sub.2 H.sub.2, OPO.sub.3 H.sub.2. Also included are radiation or chemically grafted perfluorinated polymers, in which a perfluorinated carbon chain, for example, PTFE, fluorinated ethylene-propylene (FEP), tetrafluoroethylene-ethylene (ETFE) copolymers, tetrafluoroethylene-perfluoroalkoxy (PFA) copolymers, poly (vinyl fluoride) (PVF) and poly (vinylidene fluoride) (PVDF) is activated by radiation or chemical initiation in the presence of a monomer, such as styrene, which can be functionalised to contain an ion exchange group.
3) Fluorinated polymers such as those disclosed in EP 0 331 321 and EP 0345 964 (Imperial Chemical Industries plc) containing a polymeric chain with pendant saturated cyclic groups and at least one ion exchange group which is linked to the polymeric chain through the cyclic group.
4) Aromatic polymers such as those disclosed in EP 0 574 791 and U.S. Pat. No. 5,438,082 Hoechst AG) for example sulphonated polyaryletherketone. Also aromatic polymers such as polyether sulphones which can be chemically grafted with a polymer with ion exchange functionality such as those disclosed in WO 94/16002 (Allied Signal Inc.).
5) Nonfluorinated polymers include those disclosed in U.S. Pat. No. 5,468,574 (Dais Corporation) for example hydrocarbons such as styrene-(ethylenebutylene)-styrene, styrene-(ethylene-propylene)-styrene and acrylonitrile-butadiene-styrene co- and terpolymers where the styrene components are functionalised with sulphonate, phosphoric and/or phosphonic groups.
6) Nitrogen containing polymers including those disclosed in U.S. Pat. No. 5,599,639 (Hoechst Celanese Corporation), for example, polybenzimidazole alkyl sulphonic acid and polybenzimidazole alkyl or aryl phosphonate.
7) Any of the above polymers which have the ion exchange group replaced with a sulphonyl chloride (SO.sub.2 Cl) or sulphonyl fluoride (SO.sub.2 F) group rendering the polymers melt processable. The sulphonyl fluoride polymers may form part of the precursors to the ion exchange membrane or may be arrived at by subsequent modification of the ion exchange membrane. The sulphonyl halide moieties can be converted to a sulphonic acid using conventional techniques such as, for example, hydrolysis.
Other polymeric materials which are not proton conducting polymers may be used in addition to or in place of a proton conducting polymer. For example, such polymers can be used for applications requiring a bipolar membrane or a completely anion exchange membrane. Anion exchange polymers are generally based on quaternary ammonium groups, rather than the fixed sulphonic acid groups in proton conducting polymers. These include, for example, the tetraalkyl ammonium group (--N.sup.+ R.sub.3) and the quaternary ammonium centre in Tosflex.RTM. membranes (--N(R.sub.1)(CH.sub.2).sub.y N.sup.+ (R.sub.3)) supplied by Tosoh. However, it can be envisaged that all of the proton exchange polymers described above could have anion exchange equivalents.
Other non-ion conducting polymeric materials may be used in addition to the one or more ion conducting or proton conducting polymers. Examples of such nonion conducting polymers include PTFE, FEP, PVDF, Viton.RTM. and hydrocarbon types such as polyethylene, polypropylene and polymethylmethacralate.
The polymer is suitably applied to the fibres in the form of a solution, the solvents of which may be either organic or aqueous based. Solvents of all of the above polymers may include or may be modified to include, water, methanol and/or other aliphatic alcohols, ethers, acetone, tetrahydrofuran (THF), n-methylpyrrolidone (NMP), dimethyl sulphoxide (DMSO) dimethyl formamide (DMF) dimethyl acetamide (DMAc) or protonic solvents such as sulphuric acid or phosphoric acid and/or mixtures of the above. However, it has been found that an essentially aqueous solution of the polymer as described in EP 0 731 520 is preferred.